Why?
The Görli testnet currently used by Threshold/Keep for development purposes is planned to become deprecated with the end of year 2023. The testnet that is planned to replace it is called Holešky, however it's not yet available - it's planned it will become widely accessible on Oct 1, 2023 (source).
Switching our infrastructure to support new testnet is quite time consuming, so moving directly from Görli to Holešky may be quite risky, especially if there would be some delays in the date of Holešky genesis (not meeting the planned timelines is not a rare occurrence in the Ethereum space). As a solution, we decided to switch first to another testnet that is currently live - Sepolia. This testnet's EOL is planned for 2026, which gives us plenty of time to move to Holešky before Sepolia gets deprecated.
Testnets Lifecycle:

Read more in Predictable Ethereum Testnet Lifecycle Proposal.
How?
Deploying contracts to a new testnet and adjusting CI accordingly is a work consisting of many tasks and requiring coordination of several team members. We shouldn't get rid of Görli support until we're sure that everything works on Sepolia smoothly (maybe even until Görli gets deprecated). We may consider adding Holešky support together with Sepolia in some places, for the future use (for example in Hardhat config).
What & who?
Why?
The Görli testnet currently used by Threshold/Keep for development purposes is planned to become deprecated with the end of year 2023. The testnet that is planned to replace it is called Holešky, however it's not yet available - it's planned it will become widely accessible on Oct 1, 2023 (source).
Switching our infrastructure to support new testnet is quite time consuming, so moving directly from Görli to Holešky may be quite risky, especially if there would be some delays in the date of Holešky genesis (not meeting the planned timelines is not a rare occurrence in the Ethereum space). As a solution, we decided to switch first to another testnet that is currently live - Sepolia. This testnet's EOL is planned for 2026, which gives us plenty of time to move to Holešky before Sepolia gets deprecated.
Testnets Lifecycle:

Read more in Predictable Ethereum Testnet Lifecycle Proposal.
How?
Deploying contracts to a new testnet and adjusting CI accordingly is a work consisting of many tasks and requiring coordination of several team members. We shouldn't get rid of Görli support until we're sure that everything works on Sepolia smoothly (maybe even until Görli gets deprecated). We may consider adding Holešky support together with Sepolia in some places, for the future use (for example in Hardhat config).
What & who?
keep-common(Add Sepolia to the list of supported networks keep-network/keep-common#118)@keep-network/keep-core- Support deployment of v1 contracts on Sepolia keep-core#3712@keep-network/keep-ecdsa- modified locally (repository deprecated)@keep-network/tbtc- modified locally (repository deprecated)@threshold-network/solidity-contracts- Support deployment on Sepolia #151, Add missing${{ }}markers in thecontracts.yml#152, Upgradehardhat-deployplugin to0.1.37version #154@keep-network/coverage-pools- Support deployment on Sepolia keep-network/coverage-pools#235@keep-network/random-beacon- Support deployment on Sepolia keep-core#3706@keep-network/ecdsa- Support deployment on Sepolia keep-core#3706@keep-network/tbtc-v2- Support deployment on Sepolia tbtc-v2#691@keep-network/tbtc-v2.ts- Support deployment on Sepolia tbtc-v2#691@keep-network/tbtc-v2-arbitrum(nice to have) - Add Sepolia / Arbitrum Sepolia support for Arbitrum cross-chain tbtc-v2#753@keep-network/tbtc-v2-optimism(nice to have) - Add Sepolia / Optimism Sepolia support for Optimism cross-chain tbtc-v2#752@keep-network/tbtc-v2-polygon(nice to have) - Add Sepolia support for Polygon cross-chain tbtc-v2#751@keep-network/tbtc-v2-solana? (once Solana support lands onmain) (nice to have)@keep-network/tbtc-v2-base- Add Sepolia / Base Sepolia support for Base cross-chain tbtc-v2#742SepoliaLightRelay.sol- Support deployment on Sepolia tbtc-v2#6910x68ad60CC5e8f3B7cC53beaB321cf0e6036962dBc)@keep-network/keep-core@1.8.1-sepolia.0@keep-network/keep-ecdsa@1.9.0-sepolia.0@keep-network/tbtc@1.1.2-sepolia.0@threshold-network/solidity-contracts@1.3.0-sepolia.0@keep-network/coverage-pools@2.1.0-sepolia.0@keep-network/coverage-pools@1...(There were problems with publishing package for v1, we will skip it's deployment and manage without it, at least for now)@keep-network/random-beacon@2.1.0-sepolia.1@keep-network/ecdsa@2.1.0-sepolia.1@keep-network/tbtc-v2@1.6.0-sepolia.0keep-network/tbtc-v2.ts@1.3.0-sepolia.0(published based on code from19e2dfffcommit, package marked as deprecated)@keep-network/tbtc-v2-arbitrum(also update.gitignoreandpackage.json) (nice to have)@keep-network/tbtc-v2-optimism(also update.gitignoreandpackage.json) (nice to have)@keep-network/tbtc-v2-polygon(also update.gitignoreandpackage.json) (nice to have)@keep-network/tbtc-v2-solana? (also update.gitignoreandpackage.json) (once Solana support lands onmain) (nice to have)@keep-network/tbtc-v2-basemerkle-dropconfig - Support Sepolia merkle-distribution#93SimplePreApplicationon Sepoliakeep-network/ci/config/env/sepolia.env) - Support deployment on Sepolia keep-network/ci#48keep-networkandthreshold-network):SEPOLIA_ETH_HOSTNAME_HTTPSEPOLIA_ETH_HOSTNAME_WSTESTNET_ETH_CONTRACT_OWNER_PRIVATE_KEYDAPP_DEV_TESTNET_ETH_CONTRACT_OWNER_PRIVATE_KEY.ymlfiles in following projects to use above secrets when deploying on Sepolia:threshold-network/solidity-contractskeep-network/coverage-pools(?)keep-network/random-beaconkeep-network/ecdsakeep-network/tbtc-v2keep-network/tbtc-v2/monitoring- Add support for Sepolia testnet in monitoring tbtc-v2#754threshold-network/token-dashboard- https://github.com/threshold-network/token-dashboard/pull/605keep-network/tbtc-v2-arbitrum(also update.gitignoreandpackage.json)keep-network/tbtc-v2-optimism(also update.gitignoreandpackage.json)keep-network/tbtc-v2-polygon(also update.gitignoreandpackage.json)keep-network/tbtc-v2-solana? (also update.gitignoreandpackage.json) (once Solana support lands onmain)keep-network/tbtc-v2-base(also update.gitignoreandpackage.json)dapp-developmentbranch in following projects to support Sepolia:threshold-network/solidity-contracts- [Do not merge] dApp-friendly contract deployment #118keep-network/keep-core- [Do not merge] dApp-friendly contract deployment keep-core#3118keep-network/tbtc-v2- [Do not merge] dApp-friendly contract deployment tbtc-v2#403dapp-development-sepolia-tagged packages for:@threshold-network/solidity-contracts@1.3.0-dapp-dev-sepolia.0@keep-network/random-beacon@2.1.0-dapp-dev-sepolia.0@keep-network/ecdsa@2.1.0-dapp-dev-sepolia.0@keep-network/tbtc-v2@1.6.0-dapp-dev-sepolia.0goerli- https://app.gitbook.com/o/R2meumXNNad4y1B10iL7/s/WosjlL4zUGUMlcMfuSAp/~/changes/268/resources/contract-addresses/sepolia-testnet, https://app.gitbook.com/o/R2meumXNNad4y1B10iL7/s/WosjlL4zUGUMlcMfuSAp/~/changes/326/app-development/tbtc-v2/tbtc-sdk/guides/initialize-sdkGoerliLightRelayinkeep-coreto mention also SepoliaLightRelay - Replace mentions ofGoerliLghtRelaywithSepoliaLightRelaykeep-core#3730SepoliaLightRelaycontract keep-core#3742)optimistic-mintingconfiguration (https://github.com/keep-network/optimistic-minting/pull/101)goerli-related NPM packages (we can also do a cleanup of other obsolete packages)optimistic-minting's integration tests for Sepolia contracts